Some patients receive therapy in a rehab center, while others have outpatient therapy or do their exercises at home. During the hospitalization, the patient is taught what should be done at home. "We also teach a family member who's going to be with them what they need to do," Samuelson said. "It tends to be most efficient that way. Those who get two good treatments in a day are the ones that do the best."

The joints themselves vary in design. Samuelson favors the metal on metal hip, with a large ball, because it seldom slips out of its socket and there are fewer precautions the patient must follow, such as avoiding low chairs. But ceramic heads are good too, he said. While knees are pretty standard in terms of materials, they vary in design. Some bend more than others; some try harder to create normal knee motion. The designs keep evolving.

The biggest problem for either type of joint, traditionally, has been fixation, the joints coming loose from the bone. Porous ingrowth surfaces look promising.

No improvements, though, eliminate the need for patients to use common sense and go easy on their new joints. Samuelson says the Internet has created some problems. "Patients come in with a stack of papers. 'I read about this guy who's running marathons."

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E-mail: lois@desnews.com